Ultra wide band (UWB) telecommunications systems of the impulse type are well known from the prior art. In such a system, a symbol emitted by or intended for a given user is sent using a sequence of ultra-short waves, in the vicinity of a nanosecond or a hundredth of a picosecond.
FIG. 1 diagrammatically illustrates a user's signal corresponding to a given information symbol. This signal is made up of a temporal sequence of Nf frames, each frame itself being divided into Nc elementary intervals also called chip time.
The base signal relative to a user k, called TH-UWB (Time Hopped UWB) signal, can be expressed generally by:
                                          s            k                    ⁡                      (            t            )                          =                              ∑                          n              =              0                                                      N                f                            -              1                                ⁢                      p            ⁡                          (                              t                -                                  nT                  f                                -                                                                            c                      k                                        ⁡                                          (                      n                      )                                                        ⁢                                      T                    c                                                              )                                                          (        1        )            
where p(t) is the shape of the elementary pulse, Tc is a chip time, Tf is the length of a frame with Nf=NcTc where Nc is the number of chips in an interval, the time sequence having total length Ts=NfTf where Nf is the number of frames in the sequence. The length τ of the elementary pulse is chosen to be smaller than the chip time. The sequence ck(n) for n=0, . . . , Ns−1 defines the time hop code for the user k. The sequences of time hops are chosen so as to minimize the number of collisions between impulses belonging to time hop sequences relative to different users.
On the transmitter side, the user's base signal is modulated by the information symbol, for example using pulse position modulation (PPM):
                                          s            k                    ⁡                      (            t            )                          =                              ∑                          n              =              0                                                      N                f                            -              1                                ⁢                      p            ⁡                          (                              t                -                                  nT                  f                                -                                                                            c                      k                                        ⁡                                          (                      n                      )                                                        ⁢                                      T                    c                                                  -                                  m                  ⁢                                                                          ⁢                  ɛ                                            )                                                          (        2        )            
where ε is a modulation delay substantially shorter than the chip time Tc and m=0, . . . , M−1 is position M-PPM area.
On the receiver side, the received signal is the subject of multiple time window integration. The positions of the time windows depend on the user one wishes to receive. FIG. 1B shows the multiple time window signal associated with the user k. The time windows here are calibrated on the positions of the time hops ck(n).
In general, for a given user k, if the propagation delay is subtracted, the receiver performs an integration in some time windows of the received signal rk(t), or in time windows:
w(t−nTf−ck(n)Tc), n=0, . . . , Nf−1 for the first modulation position, or I0k=∫rk(t)w(t−nTf−ck(n)Tc)dt,
w(t−nTf−ck(n)Tc−mε), n=0, . . . , Nf−1 for the (m+1)th modulation position, or Imk=∫rk(t)w(t−nTf−ck(n)Tc−mε)dt,
w(t−nTf−ck(n)Tc−(M−1)ε), n=0, . . . , Nf−1 for the last modulation position, or:IM-1k=∫rk(t)w(t−nTf−ck(n)Tc−(M−1)ε)dt, where w(t) is a bounded support function with a width slightly larger than the duration τ of the pulse p(t).
The receiver estimates the modulation position and therefore the information symbol sent, by comparing the integrated values Imk or:
                              m          ^                =                              argmax            m                    ⁡                      (                          I              m              k                        )                                              (        3        )            
Multiple time window integration of a signal r(t) generally refers to a coherent integration operation over a plurality N of disjointed time windows:
                              I          s                =                  ∫                                    r              ⁡                              (                t                )                                      ⁢                                          ∑                                  n                  =                  0                                                  N                  -                  1                                            ⁢                                                                    w                    n                                    ⁡                                      (                    t                    )                                                  ⁢                                  ⅆ                  t                                                                                        (        4        )            where wn(t), n=0, . . . , N−1, are bounded and disjointed support functions. It will be noted that in the aforementioned example, each of the values m=0, . . . , M−1 is obtained through a multiple time window integration on Nf windows.
Multiple time window integration is also used in pulsed wave radar receivers. Indeed, to determine whether a target is present in a given range bin, the receiver integrates the received signal, after having demodulated it if applicable, in a time window. In order to improve the signal to noise ratio, it is known to integrate the received radar signal coherently in a plurality of time windows spaced out over the recurrence period of the radar 1/PRF, where PRF (Pulse Recurrence Frequency) is the recurrence frequency of the radar.
A first known example of a multiple time window integrator circuit is shown in FIG. 2A.
The circuit comprises a plurality of window switches 211, 212, 213, 214, a voltage/current conversion module 220, also called transconductance block, converting the differential input voltage into a proportional current, a functional amplifier 230, two integration capacitors 241 and 242, mounted in counter-reaction between the differential outputs and inputs of the op-amp. The integrated signal appears in differential form between the outputs Vout+ and Vout− of the integrator circuit. Two switches 251 and 252, respectively mounted in parallel on the two integration capacitors 241 and 242, ensure that the integrator is reset.
The voltage signal to be integrated is applied differentially between the inputs Vin+ and Vin− of the integrator circuit. This signal is switched by the switches 211 and 213, by a multiple window logic signal
      w    ⁡          (      t      )        =            ∑              n        =        0                    N        -        1              ⁢                  w        n            ⁡              (        t        )            towards the differential inputs of the transconductance module 220.
One example of a multiple window logic signal w(t) has already been shown in FIG. 1B. Each time window wn(t) has a bounded support An, for example wn=1 if tεAn and wn(t)=0 otherwise. The time supports An are disjointed and distributed over time uniformly (example of the aforementioned radar receiver) or non-uniformly (example of the aforementioned TH-UWB receiver).
The complementary logic signal w(t) switches the inputs of the transconductance module over a same reference voltage Vref. Thus, outside the windows wn(t) one ensures that a null differential voltage is applied to the input of the module 220. The capacitors 241 and 242 integrate the output currents of the module 220. The output voltages Vout+ and Vout− are such that at the end of the multiple window function, Vout+−Vout− represents a coherent integration of the input signal on the windows wn(t), or: Vout+−Vout−=∫α(Vin+−Vin−)w(t)dt where α is a constant that depends on the circuit.
The integrated value is reset by short circuiting the capacitors 241 and 242.
The integrator circuit illustrated in FIG. 2A has a high output impedance and good performance in terms of bandwidth and gain control.
A second example of an integrator circuit with multiple time window functions is shown in FIG. 2B. Identical elements bear the same references as in FIG. 2A. Unlike the circuit from FIG. 2A, the integrator circuit of FIG. 2B has a single-pole output configuration. This configuration makes it possible to eliminate a counter-reaction control loop with a shared mode and therefore to reduce the consumption of the circuit while increasing its bandwidth. To preserve the symmetry of the assembly, in particular to balance drains during switching, the capacitor 241 is, however, kept as well as its associated reset switch 251.
The integrator circuits of FIGS. 2A and 2B do not make it possible to perform, in parallel, a plurality of multiple window integration operations. For example, such an integrator circuit does not make it possible to obtain, in parallel, the values Imk corresponding to the different positions PPM, m=0, . . . , M−1 of a TH-UWB system or to perform a coherent integration for different range bins in a pulsed wave radar system.
A first solution to perform parallel integration operations consists of providing as many integrator circuits as integration operations. For example, in the case of the TH-UWB telecommunication system described above, M integrator circuits could provide, in parallel, the values Imk for the M modulation positions, m=0, . . . , M−1. Similarly, K parallel integrator circuits can provide integration results for K distinct users.
This solution is not, however, fully satisfactory inasmuch as the integrators can have a dispersion of their characteristics, for example the transconductance gain of the module 220 or the capacitor values of 241 and 242, which can lead to different integration constants α and therefore erroneous estimates (e.g. an erroneous estimate {circumflex over (m)}). Furthermore, the multiplication of the integrators causes a greater power consumption.
The problem at the root of the invention is therefore to propose an integrator circuit with a multiple time window function, capable of carrying out a plurality of parallel integration operations, while guaranteeing good precision and low consumption.